The glass should be kept quite undisturbed, exposed to the north, and shaded from the sun. Camphor is soluble in alcohol, but not in water, while both water and alcohol have different solvent powers, according to the temperature; hence, the solid ingredients being in excess for certain conditions of solution, depending upon temperature chiefly, and perhaps electricity and the action of light also, appear as crystals and disappear with the various changes that occur in the weather.
The various appearances thus presented in the menstruum have been inferred to prognosticate atmospheric changes. The following rules have been deduced from careful study of the glass and weather:—
1. During cold weather, beautiful fern-like or feathery crystallization is developed at the top, and sometimes even throughout the liquid. This is the normal state of the glass during winter. The crystallization increases with the coldness; and if the structure grows downward, the cold will continue.
2. During warm and serene weather, the crystals dissolve, the upper and greater part of the liquid becoming perfectly clear. This is the normal state of the glass during summer. The less amount of crystallization, that is, the greater the clear portion of the liquid (for there is always some of the composition visible at the bottom), the greater the probability of continued fine dry weather.
3. When the upper portion is clear, and flakes of the composition rise to the top and aggregate, it is a sign of increasing wind and stormy weather.
4. In cold weather, if the top of the liquid becomes thick and cloudy, it denotes approaching rain.
5. In warm weather, if small crystals rise in the liquid, which still maintains its clearness, rain may be expected.
6. Sharpness in the points and features of the fern-like structure of the crystals, is a sign of fine weather; but when they begin to break up, and are badly defined, unsettled weather may be expected.
Admiral FitzRoy, in The Weather Book, writes of this instrument as follows:—“Since 1825, we have generally had some of these glasses, as curiosities rather than otherwise; for nothing certain could be made of their variations until lately, when it was fairly demonstrated that if fixed undisturbed in free air, not exposed to radiation, fire, or sun, but in the ordinary light of a well-ventilated room, or, preferably, in the outer air, the chemical mixture in a so-called storm-glass varies in character with the direction of the wind—not its force, specially (though it may so vary in appearance, only from another cause, electrical tension).
“As the atmospheric current veers toward, comes from, or is only approaching from the polar direction, this chemical mixture—if closely, even microscopically watched—is found to grow like fir, yew, fern leaves, or hoar-frost—or like crystallizations.